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Патент USA US2113198

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April 5, 1938.
- G, NONHEBEL ET AL
2,113,198
WET PURIFICATION OF GASES
Filed Feb. 4, 1935
'
2 SheetsPSheet l
‘April 5, 19538, '
G.‘NIONHEBELI.ET AL
I
2,113,198
WET PURIFICATION vOF GASES
Filed Feb. 4, 1935
2 Sheets-VSheet 2
Patented Apr. 5, 1938
‘ 2,113,193
UNITED STATES PATENT. OFFICE‘ +'
2,113,198
WET PURIFICATION OF GASES
Gordon Nonhebel and John Lindon Pearson, Nor
ton-on-Tees, England, assignors to Imperial
Chemical Industries Limited, a corporation of
Great Britain
Application February 4, 1935, Serial No. 4,891
In Great Britain February 5, 1934
4 Claims.
This invention relates to the wet-purification
of gases, and in particular to the removal of sul
phur oxides and other strongly acid gases such
as hydrochloric acid from ?ue gases and the
5 like by passing the gases to be puri?ed through a
scrubber in contact with water containing cal
cium bicarbonate and a little calcium carbonate
(Cl. 23-2)
sulphur oxides will immediately form calcium
in suspension, the liquor leaving the scrubber
being replenished by the addition of lime or chalk
If excess of lime is added to the scrubber exit
(hereinafter referred to as alkali) before recir
culation in whole or in part through the scrubber.
“
the solid matter referred to above during the
process of regeneration.
-
g
liquor, the whole of the carbon dioxide dissolved
Owing to the process of recirculation, the liquor
becomes saturated with and may contain sus
be reduced even to zero. ' Since dissolved calcium
pended solid calcium sulphite and calcium sul
bicarbonate is the principal active agent in the
phate.
absorption of the sulphur oxides, any reduction 15
in its concentration will reduce the absorptive
powers of the liquor, except insofar as fresh cal-
precipitates of calcium sulphite and sulphate
cium bicarbonate is formed when the liquor
which are formed, and for this purpose a portion
of the circulating liquor is led awayv either to
again comes into contact with the‘ flue gas.
‘
drain or to a settler or ?lter or other device
If a still larger excess of lime ‘is added to vthe 20
scrubber exit liquor, so that its pH value is raised
adapted to remove solid matter. In the latter
case the clari?ed liquor may then be returned to
the circulating system. Fresh water-is, of course,
her, free calcium hydroxide will be present in,
solution and this will react with the carbon di
added to replace losses.
oxide in the ?ue gas forming a precipitate of ' '
'
In the following description of our process, all
pH values of solutions mean the pH values of
such solutions as determined by allowing a small
portion of the solutions to settle rapidly and de
30
tially lost in the extraction from the system of '
in the liquor from the ?ue gases may be absorbed v10
in the formation of calcium carbonate and conse
quently the amount of calcium bicarbonate may
In such processes it is necessary to regenerate
the liquor as far as possible by removing the
2
carbonate and bicarbonate and the former will
be precipitated from solution. Any solid calcium
carbonate in the scrubber exit liquor will be par
above about 10 when the liquor enters the‘scru‘be
calcium carbonate which will form a scale on the
scrubbing surfaces.
‘
‘
'4
‘
'
By experiment we have found that the propor
tion of calcium carbonate in' the mixedgsolids
termining its pH value colorimetrically with in
dicators by means of apparatus called capillators,
marketed by British Drug Houses Ltd. Where
of ash, calcium sulphite and sulphate‘ rejected 30
from the system can be kept below 5% by close
reference is made to a pH instrument or recorder,
this instrument has been set or standardized on
plant liquors by capillator tests as above. ' The
scrubber exit liquor is maintainedwithin the
temperature of the circulating liquor in the
is maintained within the limits 6.0 to 6.3). 'If
this control is maintained, the alkali addition
does not exceed 10% eXcess‘of-the theoretical
required by the sulphur oxides absorbed by the
liquor in the scrubber, and in the case of treat 40
ment of ?ue gases containing appreciable
amounts of alkaline dust,‘such as gases from
scrubber is usually in the range 40° C. to 60°
C.
During the process of settling and mixing
with the coloured indicator for the above men
40 tioned tests the test portion of the solution be
comes cooled to about 20° to 30° C. and the pH
values above referred to are at temperatures
within this range.
For reasons of economy th?amount of lime
F or chalk used should not greatl
,
retical amount required to ?x the sulphur oxides
control of lime addition so that the pH of the
limits 6.0 to 6.4 (or by close control of chalk
addition so that the pH of the scrubber e?luent
35
boilers ?red 'with'powdered fuel or gases from
cement kilns, thealkali to be added may be subj-_
stantially below theoretical. We have found that 45
it is desirable to operate the process with a
absorbed by the liquor, while the liquor in_ the
slight excess of calcium carbonate in suspension
scrubber should not be allowed to become strong
ly acid (pH 5 or under) owing to the corrosion
which would then tend to take place and owing
to the diminished absorption e?iciency of the
liquor for the sulphur oxides. It will be evident
from the nature of the gases under treatment,
that any lime or hydrated lime added to the
in the liquor as this excess prevents formation
of substantial acidity in the event of a sudden
its solubility in water is materially increased with
reduction in the pH value of the solution, and
liquor as neutralizing agents for the absorbed
this increase for a given change in pH value is 65
increase in the acidity of the ?ue gas under 50
treatment.
'
'
V
_
It is also a property of calcium sulphite that
2
2,118,198
greater the lower the pH range. During the
passage of the liquor through. the scrubber, the
pH of the solution falls. For example in a par
ticular type of scrubber with which we have made
experiments, the change in pH on full load is:-—
From 6.8 at the entrance to 6.2 at the exit, with
lime neutralization; and from 6.5 at the entrance
to 6.1 at the exit, with neutralization by a re
active ?nely divided chalk, such as industrial pre
through a liquor cell containing a calomel elec
trodeand a glass electrode dipping into the scrub
ber exit liquor with a second calomel electrode
In order therefore to obtain. complete freedom
from scaling by calcium sulphite in the scrubber,
making contact with liquor of constant known pH
contained inside the glass electrode. The pH 10
is then measured by measuring the electromotive
force between the two calomel electrodes by
we arrange the gas and liquor rates vso that the
means of a thermionic potentiometer.
cipitated by-product chalk.
amount of calcium sulphite formed during each
15 passage of the liquor through the scrubber is not
greater than the sum of
(l) The increase in solubility of calcium sul
phite in the liquor during its passage through the
scrubber, due to the change in pH. We have
found that this increase is about 1.5 milligram
equivalents of calcium sulphite per litre for a pH
change from 6.8 to 6.2 in scrubber liquors, and
(2) The maximum permissible amount of su
persaturation of the liquor by calcium sulphite
.25 that is tolerated by the material of the scrubber
packing without the growth of scale.
This
amount of supersaturation is about 4 milligram
equivalents of calcium sulphite per litre of scrub
ber liquor.
30
usual coloured indicators such as the capillators
already mentioned, but preferably we determine
the pH of the solution continuously by passing a
portion of the liquor through the cell of a hy
drogen ion measuring arrangement, for example
It follows from the considerations described in
this speci?cation that the maximum capacity of
the scrubber without scaling and without waste
of alkali is obtained by proportioning the alkali
addition to the scrubber exit liquor so that the
.35 latter is maintained as closely as possible at pH
6.2 for lime addition or 6.1 for chalk addition.
In practice we ?nd it necessary to allow for
increases or decreases of 0.1 pH or 0.2 pH depend
ing on whether chalk or lime respectively is em
ployed as alkali.
The object of the present invention is to pro
vide a simple and. convenient method of regu
lating theaddition of the alkali in a process of
the kind described in order to obtain maximum
4.5 economy of alkali, freedom from scaling of the
scrubber packing by calcium carbonate or cal
cium sulphite, maximum capacity of the scrub
ber, maximum capacity of the solids removal plant
(namely, settlers or ?lters or the like) and free
50 dom from corrosion.
According to our inventiomwe determine the
pH of the scrubber exit liquor continuously or at
very frequent intervals, and we regulate the ad
dition of the lime or chalk so that this pH is
55 within the limits: 6.0 to 6.4 and preferably at 6.2
with lime neutralization or 6.0 to 6.3 and prefer
In one
form of this instrument, adjustment can be made
to the potentiometer so that no current is ?owing 15
when the pH of the scrubber exit liquor is at the
required value. Then under out-of-balance con
ditions current will flow one way or the other,
and this can be registered on a continuously re
cording milliammeter and may also be made to 20
operate relays which in turn will open or close
the alkali supply valve as required.
A suitable arrangement for this purpose is il
lustrated in the accompanying drawings, in which
Figure I is a diagrammatic ?owsheet of a ?ue gas 25
washing plant, Figure II is a diagrammatic ar
rangement of a cell for the continuous measure
ment of hydrogen ion values of the liquor, Figure
III isa wiring diagram of a suitable thermionic
potentiometer, and Figure IV is a diagrammatic 30
arrangement of the automatic control for the al
kali supply valve.
Referring to Figure I, the flue gas enters the
scrubber 3 at I and leaves at 2. The liquor leav
ing the scrubber well 4 is introduced at 6 into a 35
U-shaped tank 5, the object of which is to pro
vide such a capacity in the liquor recirculating
system that sufficient time, e. g. 2-3 minutes, is
allowed for crystallization of calcium sulphite and.
sulphate formed in the system. After leaving the 40
tank 5 the liquor is recirculated to the scrubber by
the pump 7. In order to control the pH auto
matically, a small sample is bled off continuously
to the liquor cell 8 of a pH recorder. The liquor
after passage through the liquor sampling cell
can either be rejected to drain or returned to the
circulating system, for example by passage to the
settler 9.
In order to remove accumulated solid matter a
portion of the liquor leaving the scrubber is
purged off at In and is passed to the settler 9.
A part of the clari?ed liquor from this settler
is passed to the tank vll, from which liquor is
continuously extracted by pump l2 and returned
to the scrubber via valve H. The thick liquor
ably at 6.1 with chalk neutralization. Preferably
from the settler 9 is dewatered on a rotary ?lter
l4 and the ?ltrate is added to the tank I I.
we use a continuous pI-l recorder for controlling
the addition of alkali so that the scrubber exit
the settler 9 is passed by the line l5 to a lime or
60 liquor has a substantially constant pH Within the
speci?ed range,
The lime or chalk should be added continu
ously, and is most conveniently added in the form
of a slurry.
The valve controlling the ?ow of
65 the alkali may be manually ‘operated or con
nected through known regulating mechanism to
the pH recorder so that automatic control is ob
tained. The lime or chalk may be added to'the
liquor before or after the point at which a por
70 tion of the liquor is withdrawn for removal of
solid matter, or it may be added to the clari?ed
liquor which is returned to the system, 'or‘to- the
make-up
water.
‘
‘
‘
r
.
When determining the pH of the solution man
75 ually and at frequent intervals we employ the
.
The other part of the clari?ed liquor from
chalk slurry preparation tank IS. The slurry is
withdrawn by a pump l1 and passed via a valve
l3 and line H! into the circulating stream of
liquor. The valve I8 is controlled by the mecha
nism illustrated“? Figure IV in accordance with
the pH of the , vdear leaving the scrubber well 4 65
via cell 8.
Figure 11 illustrates one method of sampling
the‘liquor continuously. The liquor enters the
cell 8 through a. pipe 2|, flows past measuring
electrodes 22 and 23 and passes out at 24. 22 is 70
a saturated calomel electrode dipping into the
sample of liquor. 23 is a glass electrode also
dipping into the scrubber liquor. The bulb of
the glass electrode 23 is ?lled with a buffer solu
tion of known pH, making contact with a second
2,118,198
similar calomel electrode 25.
The electromotive
force. of this arrangement is given by the equa
tion E total=E glass+0.0001984 T (pH l-pH 2)
where E glass is the asymmetric potential of the
glass electrode. T is the absolute temperature in
degrees centigrade; pH 1 and pH 2 are the pH
values of the scrubber liquor and the buffer solu
tion respectively. This electromotive force is
measured by taking wires 26 to the terminals of
thermionic potentiometer of the type illustrated
10
in Figure III.
A simple form of circuit for a thermionic poten
tiometer is shewn in Figure III, which comprises
a Wheat-stone bridge containing ?xed resistances
15 3| and 32 and two thermionic tubes 33 and 34 36
is the ?lament battery, 35 the grid bias and 3'! the
plate battery. The unknown potential from the
liquor cell in Figure II is led to the terminals 26
where it is superimposed on the grid bias 35. The
instrument 38 measures the out-of-balance cur
3
tained the nature of our said invention and in
what manner the same is to be performed, we
declare that what we claim is:1. In a process of ?ue gas washing with the aid
of a recirculated liquor to which a slurry of lime
is continuously added in amount approximately
equivalent to the sulphur oxides and other acid
gases absorbed from the ?ue gas, the steps of con
tinuously determining the pH of the scrubber
exit liquor by measuring the E. M. F. between an 10
electrode sensitive to changes in pH and a stand
ard reference electrode, both of said electrodes
being immersed in said liquor and being without
effect on the pH thereof, and regulating the addi
tion of said slurry so as to keep said pI-I Within 15
the limits 6.0-6.4.
2. In a process of ?ue gas washing with the aid
of a recirculated liquor to which a slurry of lime
is continuously added in amount approximately
equivalent to the sulphur oxides and other acid 20
rent from the Wheatstone bride. This instrument
may be a recording milliammeter, to which is at
tached a relay for the automatic control of the
alkali valve operating motor. A simple form of
25 such relay is shewn in ‘Figure IV.
Referring now to Figure IV, 41 is a metal needle
gases absorbed from the ?ue gas, the steps of
passing a portion of the scrubber exit liquor
through a cell containing a glass electrode and a
limb of a calomel electrode, measuring the
E. M. F. between said electrodes, and regulating 25
swinging between two metal segments 42 and 43,
E. M. F. within limits corresponding to a pH of
said exit liquor of 6.0-6.4.
3. In a process of ?ue gas washing with the aid
of a recirculated liquor to which a slurry of chalk
is continuously added in amount approximately
equivalent to the sulphur oxides and other acid
gases absorbed from the ?ue gas, the steps of
continuously determining the pH of the scrubber
exit liquor by measuring the E. M. F. between an 35
electrode sensitive to changes in pH and a stand
ard reference electrode, both of said electrodes
set a short distance apart on either side of the
zero of the instrument.
Thus, supposing it is
30 desired to maintain the pH between the limits 6.0
to 6.3, the galvanometer needle is set to read zero
when the pH is at 6.15, and the segments 42 and
43 are placed at the positions corresponding to
de?ections of the needle to pH 6.0 and pH 6.3
35 respectively. When the pH rises or falls to the
limits 6.0 and 6.3, connections are made through
the wires 44 and 45 or 44 and 46, which operate
the relay R up or down by virtue of the power
supplied by the battery 48. According to whether
40 the relay is moved up or down, current is passed
to the motor 49 operating the lime supply valve
l8, so that it either opens or closes.
It will be understood that, in order to be suit
able for use in practicing the invention, the elec
45 trodes employed in determining the pH of the
scrubber exit liquor should be of such type that
they are without effect upon the pH of the liquor
under test. Of such type are the electrodes spe
ci?cally referred to in the “foregoing description.
50 On the other hand, a hydrogen electrode is un
suitable, in that the necessary stream of hydro
the addition of said slurry so as to keep said
being immersed in said liquor and being without
effect on the pH thereof, and regulating the
addition of said slurry so as to keep said pH 40
within the limits 6.0-6.3.
4. In a process of ?ue gas washing with the aid
of a recirculated liquor to which a slurry of chalk
is continuously added in amount approximately
equivalent to the sulphur oxides and other acid
gases absorbed from the ?ue gas, the steps of
passing a portion of the scrubber exit liquor
through a cell containing a glass electrode and a
limb of a calomel electrode, measuring the E. M. F.
between said electrodes, and regulating the addi 50
gen removes carbon dioxide from the liquor and
tion of said slurry so as to keep said E. M. F.
within limits corresponding to a pH of said exit
hence alters its pH. A quinhydrone electrode is
liquor of 6.0-6.3.
also unsuitable on account of its reaction with
55 the sulphur dioxide contained in the liquor.
Having now particularly described and ascer
GORDON NONHEBEL.
JOHN LINDON PEARSON.
55
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